1. Field of the Invention
The present invention relates to a light source unit having a luminescent plate and a projector including this light source unit.
2. Description of the Related Art
In these days, data projectors are used on many occasions as image projection apparatuses which project images including images of screens and video images of personal computers, as well as images based on image data which is stored in memory cards on to a screen. These projectors are such that pencils of light which are emitted from a light source are collected to a display device such as a micromirror device called DMD (Digital Micromirror Device) or a liquid crystal plate so as to form an image on a screen by projecting light emitted from the display device onto the screen via a projection side optical system.
Conventionally, the mainstream of these projectors has been those which use a high-intensity discharge lamp as a light source. However, in recent years, there have been made many developments and proposals on projectors which use, as a light emitting element of a light source, a light emitting diode (LED), a laser emitter, a device utilizing organic EL technology or a luminescent material. For example, Japanese Patent No. 3415403 proposes, as a light source of a projector, three types of laser light sources which emit individually light of red, green and blue wavelength bands. In addition, Japanese Unexamined Patent Publication No. 2004-341105 (JP-A-2004-341105) proposes a light source unit including an excitation light source which emits ultraviolet light and a luminescent wheel which is a rotary luminescent plate on which luminescent material layers emitting individually light of red, green and blue wavelength bands are laid end to end in a circumferential direction.
In the light source unit proposed in Japanese Patent No. 3415403, including the three types of laser light sources requires a driving circuit, leading to a problem that the light source encounters difficulty in being incorporated in a projector which tends to be smaller in size and thinner in thickness so as to meet the current market demands. In addition, in order to realize an increase in luminance in a projector like the one described above which includes those three types of laser light sources, the respective outputs of the laser light sources need to be increased. When the outputs are increased, the respective calorific powers of the light sources are increased, and therefore, a cooling mechanism needs to be enlarged in size, leading to a problem that the light source encounters difficulty in being incorporated in a projector which tends to be smaller in size and thinner in thickness so as to meet the current market demands. Further, although it is considered that the laser light sources are increased in size in order to realize such an increase in luminance, when the laser light sources are increased in size, the following problems have to be faced. That is, the enlarged light source encounters difficulty in being incorporated in a projector which tends to be smaller in size and thinner in thickness so as to meet the current market demands, and the utilization efficiency of light of the light source is reduced due to an increased difference between the Etendue of a light source side optical system and the Etendue of a projection side optical system which results from the increased light emitting area.
On the other hand, in the light source unit proposed in JP-A-2004-341105, an increase in luminance can be realized with a difference between the Etendue of a light source side optical system and the Etendue of a projection side optical system kept suppressed to a low level by increasing the output of the excitation light source. However, when excitation light is shone normal to a flat plane of the luminescent wheel from a front side thereof, part of light incident on the luminescent material layers passes through the luminescent material layers and is then reflected on a reflecting surface of the luminescent wheel to thereby be emitted from an incident surface of the excitation light towards the excitation light source without exciting the luminescent materials, which happens occasionally, leading to a problem that the utilization efficiency of excitation light is reduced. Further, since light emitted from the luminescent materials scatters in every direction, part of luminescent light is wasted by being attenuated in the luminescent material layers, leading to a problem that the utilization efficiency of luminescent light is reduced.